Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NO.sub.x) produced during engine operation into more desirable gases. Generally the catalysts include a precious metal like platinum carried on a high surface area, porous support material like alumina (Al.sub.2 O.sub.3). Alumina is conventionally used as a catalyst support because it has high temperature stability. That is, it retains its pore structure and hence surface area at the high exhaust gas temperatures to which it which it may be exposed during automotive operation. High surface area and retention of pore structure is important because it allows maximum contact between the exhaust gases and a catalyst material like platinum. If the pore structure of the catalyst support material is not stable and collapses during use, the catalytic material carried on the surface of the support can become occluded within the support material so that it is no longer able to catalyze the conversion of exhaust gases.
While alumina desirably has high temperature stability, one of its deficiencies is that it is poisoned by sulfur oxides (SOx) in the exhaust gas stream. That is, the alumina reacts with oxides of sulfur like SO.sub.2 and SO.sub.3 to forms a sulfate, Al.sub.2 (SO.sub.4).sub.3 which is stable at high temperatures. Formation of alumina sulfate results in a decrease in the surface area and pore volume of the alumina. Hence, poisoning of the alumina by SOx lowers the efficiency of the catalyst since less catalyst is exposed to the exhaust gases.
It is known that titania is resistant to SOx poisoning at the high operating temperatures in the automotive exhaust gas system. That is because stable sulfates of titania do not exist at these high temperatures. Two major disadvantage of using titania for a catalyst support, however, are that its anatase form which is stable at low temperatures has a low surface area (25 m.sup.2 /g) and also that its anatase phase is converted to extremely low surface area (1 m.sup.2 /g) rutile phase titania at the higher temperatures experienced in the automotive exhaust (700.degree. C.). Hence, for several reasons titania is not generally considered suitable as an automotive exhaust gas catalyst support.
It would be desirable to develop a high surface area support for catalysts which is stable and retains it high surface structure at elevated temperatures and also is not susceptible to poisoning by SOx. The present invention provides such a support material.